Parents who stock their kitchens with healthy food, limit eating out and ensure that their children stay active may overlook a threat to their best efforts to keep their kids lean: the television.

As a new report from the Kaiser Family Foundation showed last week, children and teens get bombarded with thousands of food ads yearly. So many, in fact, that they add up to 51 hours of viewing time yearly for kids ages 8 to 12; nearly 41 hours for those 13 to 17 and about 30 hours among those 2 to 7.

That might not be a problem if the ads promoted nutritious fare, such as fruit, vegetables, whole grains and low-fat dairy products. But the report -- the largest ever conducted of food marketing to children and teens -- highlights how TV commercials tout mostly junk food. Candy and snacks accounted for a third of the food commercials, while 28 percent were for cereals, many of them loaded with added sugar, and 10 percent were for fast food.

Lest you think that these ads might not be having much effect, consider this: A 2006 Institute of Medicine report found that food ads and marketing strongly influence children's food preferences and their diets.

Any parent who has shopped for groceries with children probably knows firsthand the strength of this marketing effort. Science has documented that "the foods that are being advertised are the ones that children are going to be asking their parents to purchase," says Mary Story, who studies food marketing to children at the University of Minnesota in Minneapolis.

How often do parents give in to the pleas for junk food? About 50 percent of the time, according to Story's research of family members shopping for groceries together.

Fifty percent of countries now regulate food marketing to children, according to an editorial by New York University nutrition professor Marion Nestle published last year in the New England Journal of Medicine. Australia bans food advertisements aimed at children 14 and younger. In the Netherlands, food companies can't advertise sweets to kids younger than 12. Sweden prohibits the use of cartoon characters to promote foods to children younger than 12.

The United States has far fewer regulations. But the Institute of Medicine report advised that Congress enact more rules "if the industry does not change its practices voluntarily."

Since then, the Ad Council and the National Advertising Review Council have launched voluntary self-regulation efforts designed to help control food marketing and promote healthier messages to younger TV viewers.

"The Institute of Medicine and members of Congress on both sides of the aisle, including Senator Sam Brownback and Senator Tom Harkin, have sent messages that they want, and expect to see, some substantial changes in food advertising to kids," said Vicky Rideout, vice president and director of Kaiser's Program for the Study of Entertainment Media and Health, who directed the latest study. "It's also very clear that they are not prepared to mandate changes until they give the industry a chance to do something voluntarily."

Some critics say that is not enough. "There is a lot of talk of companies' commitment to addressing junk-food marketing to kids," notes Margo Wootan, director of nutrition for the Center for Science in the Public Interest, a consumer advocacy group.

"But overwhelmingly the messages promote unhealthy eating. . . . We have been very patient in encouraging the industry to do the right thing. If industry is not going to self-regulate, then the courts and Congress are going to need to step in and do it."

A 2005 survey of parents commissioned by the Ad Council illustrates how much work there is to be done. Just 21 percent said they limited the calories their children consume; only 37 percent said they knew what serving sizes were appropriate for their children. Only about half described their kids as being physically fit.

Parents also reported struggling with their own habits. Just about a third said that they eat healthy meals -- about the same proportion who reported being physically active.

Yet research clearly shows that children practice what their parents do, not what they preach. Adults who snack on fruit and vegetables or who stay physically active tend to have children who do the same.

That's why David Ludwig, director of the obesity program at Children's Hospital in Boston, advises parents and children to keep a television log. Place a notebook and pencil next to the TV. Get each family member to record when the set goes on and when it goes off.

"The whole family needs to do this," Ludwig says, "because just monitoring raises consciousness about how much television is being watched."

Also, limit each person to no more than two hours per day, he advises, noting that watching just "one hour daily is better, one half-hour is best."

Not many calories are burned watching TV. In fact, research shows that metabolism actually declines to levels as low as during sleep.

Plus, the flurry of food commercials can help stoke hunger and encourage snacking. In a two-year study of 500 middle-school children, Ludwig found that kids consumed an additional 167 calories for every hour of television they watched.

Nearly all of these extra calories came from soft drinks, french fries, salty snacks, cookies, candy and fast food -- "Precisely the foods that are most heavily advertised on television," Ludwig notes.

So if you and your kids snack while watching television, reach for fruit, carrot sticks, bean dip, salsa and other healthier fare.

And while you're at it, try to get off the couch at least during the commercials. Or consider putting some exercise equipment in the family room near the television.